How much paint do I need? Calculate surface area and coverage

Buying too little paint is frustrating: you run out halfway through the job, or you end up applying a layer thinner than you should. Buying too much costs you money unnecessarily, especially with antifouling and two-component products that don't have a long shelf life once opened. With a few formulas, you can calculate fairly accurately beforehand how much you need. We explain how it works and work through an example.

Step 1: calculate the surface area

A hull is not a rectangle, so we use rules of thumb. These provide an approximation that is more than sufficient for determining the amount of paint needed. You need the following measurements:

• LOA: length overall
• LWL: length at the waterline
• B: maximum beam
• D: draft
• Freeboard: average height from the waterline to the deck edge

Underwater hull

Surface area = LWL × (B + D) × factor

The factor depends on the hull shape:

Above-water hull

Surface area = 2 × (LOA + B) × freeboard

Deck

Surface area = 0.75 × LOA × B − surface area of the superstructure

Deduct the floor area of the cabin, wheelhouse and hatches if you are not painting them with the same paint.

Superstructure

For a cabin or wheelhouse, calculate the walls and roof separately: perimeter × height for the walls, plus length × width for the roof. Deduct large windows and portholes.

Step 2: how many m² per liter?

Every technical data sheet states the theoretical coverage: the number of square meters you can paint with one liter at a specific dry film thickness. This figure is calculated from the volume solids percentage and the dry film thickness (DFT):

Theoretical coverage (m²/L) = volume solids % × 10 ÷ dry film thickness in µm

A paint with 50 percent volume solids, applied to a dry film thickness of 50 µm, therefore gives 50 × 10 ÷ 50 = 10 m² per liter. If you want thicker layers, the coverage decreases proportionally.

In practice, you will never achieve that theoretical figure completely. Paint remains in the container, roller, and brush, a rough or porous surface absorbs more, and you apply it a little thicker here and there. Therefore, calculate for waste:

• Brush and roller: an additional 10 to 15 percent
• Spraying: easily 30 percent or more, depending on the method, the wind, and the shape of the object

Step 3: the number of coats

Multiply the surface area by the number of coats prescribed by the paint system. Some general guidelines:

• Antifouling: usually 2 coats, plus an extra coat on the waterline, rudder, and the front of the keel and bow, where wear is greatest.
• Topcoat on the above-water hull: 2 to 3 coats, often over one or two coats of primer.
• Epoxy osmosis prevention: 5 to 7 coats.
• Clear varnish outdoors: 5 to 8 coats.

Unsure which products and coats are right for your boat? Then use our paint guide.

Calculation example: 9-meter motorboat

We take a motorboat with the following dimensions: LOA 9.00 m, LWL 8.00 m, beam 3.10 m, draft 0.80 m, and an average freeboard of 1.00 m.

Underwater hull with antifouling

Surface area: 8.00 × (3.10 + 0.80) × 1.0 = 8.00 × 3.90 = 31.2 m².

Suppose the antifouling has 55 percent volume solids and is applied to a dry film thickness of 50 µm per coat. The theoretical coverage is then 55 × 10 ÷ 50 = 11 m²/L.

1. Two coats: 31.2 × 2 = 62.4 m²
2. Theoretically required: 62.4 ÷ 11 = 5.67 liters
3. Plus 15 percent waste (roller): 5.67 × 1.15 = 6.5 liters

With three 2.5-liter cans (7.5 liters in total), you will have enough for two full coats and approximately one liter left over for the extra coat on the waterline, rudder, and other high-wear areas.

Above-water hull with topcoat

Surface area: 2 × (9.00 + 3.10) × 1.00 = 2 × 12.10 = 24.2 m².

In this example, the topcoat has 50 percent volume solids and is applied to a dry film thickness of 40 µm per coat: 50 × 10 ÷ 40 = 12.5 m²/L.

1. Two coats: 24.2 × 2 = 48.4 m²
2. Theoretically required: 48.4 ÷ 12.5 = 3.87 liters
3. Plus 15 percent waste: 3.87 × 1.15 = 4.5 liters

Don't forget that for a complete system you will also need primer or undercoat. You calculate this in the same way using the data from that product's data sheet.

The figures in this example are calculated values to demonstrate the method. The actual coverage, dry film thickness, and number of coats are stated in the technical data sheet of the product you use, and those always take precedence.

Prefer not to calculate it yourself?

With our paint calculator, you enter the dimensions of your boat and immediately get an estimate of the surface area and the amount of paint required. You can find the technical data for each product in the data sheets.

Need help?

Would you like us to take a look at your calculation, or are you unsure about the right paint system? Contact HOAC Marine. We are happy to help you with the calculations.